The present invention relates to improved apparatus for thermal treatment of semiconductors in a treating gas.
As a consequence of recent trends, there has been a gradual tendency for semiconductors to be processed as wafers of larger dimensions, whereupon wafers as large as 5 inches, 127 mm, in diameter may be encountered in some instances. Hence, it has become necessary for semiconductor treating tubes to be increased in size to outer diameters of 150 to 160 mm. Wafer-carrying boats also have been increased in size to lengths as long as 800 mm.
In semiconductor treating tubes, it is known for baffle plates to be used to prevent intrusion of an external gas, to utilize an internal gas effectively, and to distribute an internal gas uniformly, but a sealing cap ordinarily is attached to a treating tube so as to prevent escape of an internal gas, and so as to prevent intrusion of an external gas.
In thermal treatment of a wafer in a treating tube having a heating portion, which is heated to high internal temperatures by a heating element surrounding the heating portion, a boat carrying the wafer may have to be inserted through a region having a steep gradient of temperature within the heating portion of the treating tube. Alternatively, the boat carrying the wafer may have to be withdrawn through a region having such a gradient. In either instance, cracks or pinholes may be formed in the wafer by thermal stress, and there may be other problems, such as a short circuit or thermal breakage of the treating tube at its end portions exposed to the outside atmosphere.
From prior efforts to eliminate these problems, it is known for an extending portion of the treating tube to protrude longitudinally from a heat-insulating member, which surrounds the heating portion of the treating tube, in a direction of discharge of a treating gas flowing from an inlet of the treating tube to an outlet of the treating tube, so as to smoothen the temperature gradient, and so as to enable a wafer to be preheated, cooled, or both in the extending portion of the treating tube.
It is known for a useful region within a treating tube, which is heated along a heating portion as noted above, to exhibit a uniform temperature for a given dimension measured longitudinally along the treating tube. The longitudinal dimension of the useful region exhibiting a uniform temperature limits the semiconductor-treating capacity of the treating tube. However, if the treating tube has been provided with an extending portion as noted above, there heretofore has been an abrupt reduction in the longitudinal dimension of the useful region exhibiting a uniform temperature, whereupon the extending portion of the treating tube heretofore has had a characteristic disadvantage, as there has been a similar reduction in the semiconductor-treating capacity of the treating tube.
For reasons explained below, eliminating the aforementioned disadvantage by changing the quantities of heat generated at the opposite ends of the heating portion of the treating tube is not possible.
Ordinarily, temperature reductions within the treating tube are larger at opposite end parts of the heating portion of the treating tube than at a central part of the heating portion of the treating tube, since heat losses are extreme at the opposite ends of the heating portion of the treating tube. Accordingly, it is known for the quantities of heat generated (i.e. electric power) to be increased at opposite end parts of the heating portion of the treating tube. Since heat losses are greater at the treating gas outlet end where the treating tube has an extending portion, it is known for the quantity of heat generated at the treating gas outlet end to be increased above the quantity of heat generated at the treating gas inlet end. However, it has been found that, even at a uniform temperature of 1000.degree. C., a local region of lower temperature (dip) appears in the temperature gradient, near the treating gas outlet end.
Accordingly, the longitudinal dimension of the useful region exhibiting a uniform temperature is shortened, because the local region of lower temperature is excluded.